Since Aserinsky and Kleitman applied brain wave examination in the 1950s to distinguish a special state of sleep – heterophasic sleep – the staging of sleep and the physiological, biochemical and endocrine aspects of each sleep cycle have been understood in more detail. It is now known that sleep is not a complete inhibition of nerve cells, but a change of activity: one part is deactivated and the other part is activated, it is an active process of regulation and reintegration. According to the changes in EEG, EMG and eye movements, the sleep cycle can be divided into wakefulness, none rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. NREM sleep can be further divided into stages 1, 2, 3 and 4, of which stages 3 and 4 are dominated by slow-wave activity, so they are also called slow-wave sleep. In normal adults, NREM sleep is first entered at the beginning, from stage 1 through stages 2, 3, 4, and then back to stages 3 and 2, about 60 to 90 minutes after falling asleep, the first REM sleep, which lasts about 1 to 5 minutes, is called a sleep cycle. Then, it enters the second sleep cycle of NREM stage 2, and so on alternately. There are about 4 to 6 cycles of sleep throughout the night, and each cycle lasts an average of 90 minutes. Slow wave sleep is more frequent in the first 1/3 of the night, and the duration of REM sleep gradually increases, with one REM sleep lasting up to 20 minutes or more in the second half of the night. The ratio of NREM to REM varies with age, with neonatal REM sleep accounting for about 50% of the total night’s sleep. In adults, NREM accounts for about 75% to 80% of total sleep time, while REM accounts for only 20% to 25%. In older adults, REM sleep is even less.